Dark blood preparation (DB-prep) also known as “Double Inversion Recovery” (double IR) magnetization preparation, is commonly used in cardiac and vascular MRI (magnetic resonance imaging) for reducing or eliminating MRI signal data associated with blood. The known method relies upon effective inversion of the intra-cardiac or intra-vascular blood signal by the application of a pair of inversion pulses, initially a non-selective inversion pulse is followed immediately by a slice-selective inversion pulse where the slice to be imaged is re-inverted. The effect of this pair of inversion pulses is to invert the blood outside the slice to be imaged and to leave the imaged slice effectively untouched as if it had not experienced any change from its original condition. The inverted blood recovers its longitudinal magnetization during the inversion time and the slice is then imaged using one of a variety of MRI pulse sequences, typically a variant of the Turbo Spin-Echo (TSE) type. During this inversion time the blood which has experienced both inversion pulses has typically moved out of the slice and been replaced by inverted blood, as illustrated in FIG. 1. The inversion slice is usually thicker than the imaged slice (e.g. twice as thick). This is to ensure that the imaged slice is untouched even in the setting of an imperfect registration between DB-preparation slice and imaged slice.
A known system acquires imaging data (also known as data readout, RO) during the period where the longitudinal magnetization of the inverted blood is passing through a zero point and cannot contribute any signal to the image. This is called blood nulling. A representative timing of the electro cardiogram (ECG), recovery curve of the longitudinal magnetization MZ of the blood, DB-preparation module, and time of data readout relative to the recovery curve and ECG are illustrated in FIG. 2. However the inversion time required for complete blood nulling is dependent on the T1-relaxation time of the blood, which is predictable and largely constant, as well as the time between the repetitions of the DB-preparation module (specifically the non-selective inversion within the DB-preparation) which, in turn, is dependent upon the heart rate of the patient.
In a known method for acquiring images in cardiac MR, the DB-preparation module is played repeatedly in a periodic manner as data is collected in small segments over several heart beats where the imaging sequence is synchronized with an ECG, so that the heart is spatially positioned substantially identically for the collection of each segment of data. For optimal timing, the shorter the time between two consecutive DB-preparation modules, the shorter the effective inversion time to null blood. The time between the DB-preparation modules depends on a trigger pulse which determines whether single R-waves (trigger pulses 1), alternating R-waves (trigger pulses 2), or every nth R-wave (trigger pulses n) are used for triggering. The trigger pulse is usually adjusted by a scanner operator as a function of patient heart rate. While the theoretical timing for inversion and readout are predictable using known equations for simulation of the MR signal, the practicality of manually changing the sequence timing dependent on heart rate means that for most situations operators live with sub-optimal blood nulling, or spend a lot of time trying to optimize the blood nulling for the patient heart rate at the expense of scanning efficiency.
In dark-blood preparation, the correct repetition time TR (unlike in MR physics, TR in this document is defined as the time from the start of the DB-preparation module until the end of the readout, as shown in FIG. 2) is determined as the normal TR values achieved with patients within the normal range of heart rates are sufficient to allow inversion times encompassed within one RR interval to result in reasonably good blood nulling (the RR interval is the time from one R-wave to the next, i.e., the heart beat duration). This is not the case, however, in patients with very fast heart rates and especially if a third inversion pulse, the STIR pulse, is used to prepare the magnetization of an imaging slice. The third IR pulse might be played, for example, to impart a T1 plus T2 contrast and is commonly called Short tau Inversion Recovery (STIR). The inversion time of the STIR pulse is chosen to null fat and to impart a T1 contrast to the image. This can be achieved by use of a selective or non-selective inversion preparation. In known systems selective preparation is used for the STIR pulse to avoid compromised dark blood preparation and the timing with a slice-selective preparation is the same as if no STIR pulse were used.
In known dark-blood preparation (DB prep) shown in FIG. 2, blood is black or dark because the magnetization of the blood that recovers exponentially is imaged at the point in time when the recovery curve is at about zero (“nulled”). That is where the data readout starts. Zero magnetization means zero MR signal which is depicted as black in an image. Very positive or very negative magnetization both show up bright in an MR image. The magnetization assumes values from −100% of M0 to +100% of M0. M0 is the magnetization of the patient (blood and tissue) due to a strong magnetic field created by an MRI scanner. Dark blood preparation comprises inverting the magnetization outside a prepared slice so that it can recover according to the exponential recovery curve of FIG. 2. The tissue and blood inside the slice is re-inverted substantially immediately after the non-selective inversion so that it is left magnetically unaltered. It is at +100% of M0, whereas blood is at about −100% and recovers back to +100% over time. At the time of readout the blood that was in the slice during the DB preparation has left the slice due to cardiac contraction expelling blood, and the blood from outside the slice has moved inside. Now in this slice blood and tissue are magnetized differently. By timing readout, the blood magnetization is zero, hence blood appears dark and contrast between blood and tissue is emphasized. A system according to invention principles addresses the problems involved in providing pulse sequence timing in Dark blood MR imaging applications.